1. Field of the Invention
The present invention relates to a writing technique of a mask using an electron beam or laser beam, particularly to a method of generating writing pattern data of a mask for use in an extreme ultraviolet (EUV) lithography technique utilizing soft X-rays, and a method of writing a mask.
2. Description of the Related Art
In recent years, as the next-generation lithography technique, an EUV lithography technique involving a reduced reflective-type projection exposure technique using 5- to 15-nm soft X-rays has been developed on a worldwide scale. In this lithography, since there is no substance (material) suitable for a refractive optical element in an EUV light region, a mask, illumination optics, and projection optics are all constituted of reflective types. An Mo/Si multilayered film indicating a high reflectance with respect to EUV light is formed on the mask, and a Cr— or Ta-based absorber with respect to the EUV light is used in an opaque material. Similarly, the Mo/Si multilayered film is formed on a reflective mirror which is an optical element of reflective optics.
The use of a system similar to a so-called optical scanner in an EUV exposure tool is planned, illuminative light having an annular shape is applied to the mask in an oblique direction at an angle of incidence around 6°, the mask and a substrate (wafer) to be exposed are scanned relatively with respect to the projection optics at a speed ratio in accordance with the reduction ratio, and reflective light from the mask surface on which a mask pattern is formed is projected/reduced to thereby form a mask pattern into a film on the wafer.
In the projection optics of the reflective type exposure system, since a non-telecentric system is constituted on a mask side, image shift of the projected/reduced pattern on the wafer, which is in-plane displacement, is a problem. A position where the pattern is formed into an image shifts in a horizontal plane of the wafer because of unevenness of the mask surface.
For example, when a vertical position of the mask in a portion where a certain pattern is formed displaces from a reference plane by 0.1 μm, a projection image forming position shifts from an original image forming position by about 2.6 nm on the wafer. In the same manner as in a usual photo-mask, a positional shift by elastic deformation of the mask also raises a problem. The positional shift of the pattern is caused by the weight of the mask, stresses of various thin films formed on the mask (multilayered film, absorber, buffer, etc.), temperature, and holding. Since a film structure of the EUV mask is complicated, there is a possibility that the pattern positional shift by in-plane stress non-uniformity of the film raises a problem.
Displacement of the mask plane (or height) in the Z-direction (vertical direction), which is a cause for the image shift, is caused not only by an uneven shape (e.g., uneven surface) of the mask but also flatness of a mask holding mechanism (mask chuck). This also applies to unevenness of the wafer, or flatness of the wafer chuck. Furthermore, mechanical fluctuation in the Z-direction in driving the mask or a wafer stage is also a cause. Thus, since the image shift is attributed to a mask front surface shape at exposure, and does not indicate a constant value, it is difficult to control the shift by the exposure tool. However, the above-described image forming position shift of the mask pattern is minimized in a possible range, and transfer position precision needs to be secured. For this purpose, the distance between points in at least a plane between the mask and the wafer is kept constant, and exposure is performed. In a conventional exposure technique, for securement of a depth of focus (DOF) and suppression of magnification error, postures of the mask and the wafer are usually controlled by Z-axis driving mechanisms and tilt mechanisms to hold constant distances based on the result of measurement of both vertical positions using focus sensors, respectively. For example, as a method of correcting the image shift at the exposure to thereby perform exposure, in Jpn. Pat. Appln. KOKAI Publication No. 11-219900, it has been proposed that while adjusting relative positions in the Z-direction, the mask and the wafer stage should be moved synchronously in reverse along the Y-direction to thereby suppress the positional shift attributed to Z-displacement of the mask. In U.S. Pat. No. 6,229,871, it has been proposed that unevenness should be disposed on a non-planar chuck surface, and an easily bendable mask should be chucked. Accordingly, the relative positions in the Z-direction are easily driven/controlled to thereby correct in-plane positional shift in the scanning direction.
However, since the vertical position of the mask or the wafer monitored by the focus sensor indicates a value in a local position, the distance between the mask and wafer is simply controlled to be constant based on the average vertical position in an irradiation region plane even in a full field exposure tool or a scanner exposure tool. Since there is necessarily local unevenness in the irradiation region plane of a finite region, not all vertical positions in the irradiation region plane take constant values. Therefore, even when the distance between the mask and the wafer surface is controlled to be constant based on the average vertical position in the irradiation region plane, image shifts are produced in individual patterns in accordance with the local unevenness included in the irradiation region plane, and therefore positional precision is deteriorated.
As described above, there has been a problem that a transfer pattern cannot be obtained with sufficient positional precision in a non-telecentric exposure system represented by the EUV lithography technique. Therefore, there has been a demand for realization of a method of generating writing pattern data capable of obtaining a transfer pattern with the necessary positional precision, and a method of writing a mask in a non-telecentric exposure system using a reflective mask.